In a letter to the UN Security Council dated June 27, 2025, the United States presented its international legal justification for its June 22nd strikes on the Iranian nuclear facilities at Fordow, Natanz, and Isfahan—the first such U.S. military strikes on Iranian territory.
Having helped draft such letters on the use of force to the Security Council, I find the explanation inadequate. The pertinent international law argument advanced by the United States in this so-called “Article 51 letter” is unconvincing. The other legal claims and references presented by the United States are legally truly irrelevant. Executive branch lawyers need to do more on the front end to shape use of force decisions by the President to ensure they comply with both domestic and international law.
Legal Background
Article 2(4) of the UN Charter prohibits “the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.”
Article 51 of the Charter in turn specifies, in relevant part, that
Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council
In other words, notwithstanding the prohibition on the use of force imposed by Article 2(4), the forcible measures in individual or “collective self-defense” (coming to the aid of another country exercising its right of self-defense) are permissible and must be reported to the UN Security Council. Any such use of force in self-defense is constrained by the requirements of necessity and proportionality.
Importantly, the text of the Article 51 requires an “armed attack” as a prerequisite for the use of force in self-defense. Although the United States and other States have endorsed the concept of anticipatory self-defense in response to an imminent threat of armed attack, some policy experts have advocated for States being able to use force in preventative self-defense against more inchoate threats. Consistent with the weight of State opinio juris and expert opinion, the U.S. government has never adopted this theory of preventative self-defense.
Indeed, in 1981, the United States joined a unanimous UN Security Council resolution in condemning a preventative Israeli attack on an Iraqi nuclear facility at Osirak. In the view of the United States, Israel’s actions violated Article 2(4) due to the “absence of any evidence that Iraq had launched or was planning to launch an attack that could justify Israel’s use of force. … [T]he presence in a State of the military capacity to injure or even to destroy another State cannot itself be considered a sufficient basis for the defensive use of force.”
Moreover, despite some misunderstanding to the contrary, the United States decidedly did not rely on preventative self-defense as the international law justification for the 2003 invasion of Iraq, but instead claimed authority to invade Iraq on the basis that Iraq materially breached a Security Council ceasefire resolution.
An Unconvincing Claim of Collective Self-Defense
Consistent with reporting obligations under Article 51 and longstanding U.S. government practice of making these submissions, the June 27th letter is the United States’ official report on the U.S. attack on Iran. The load bearing element of the United States’ letter is that “on June 22, 2025, the Armed Forces of the United States exercised the inherent right of collective self-defense” by attacking Iran. As the letter elaborates:
In close coordination with the Government of Israel, the United States has taken necessary and proportionate action — directed solely against the Iranian nuclear program — to defend Israel and also to protect our own security, citizens, and interests, consistent with Article 51 of the Charter of the United Nations. The strikes taken on June 22 occurred because peaceful measures were exhausted. In recent weeks and months, and as it has for well over a quarter century, the United States made every effort to secure a diplomatic resolution ensuring peace, prosperity, and security for all States in the region; to prevent a dangerous spiral of nuclear proliferation and instability, such a resolution must include ending the Islamic Republic of Iran’s potential development of a nuclear weapon. However, Iran refused to negotiate in good faith or to cease enriching uranium beyond the threshold needed for peaceful, civilian power generation. (emphasis added)
There are several holes in the claim that the United States use of force was an exercise in lawful collective self-defense on behalf of Israel.
As a threshold matter, the letter fails to establish that the use of force was necessary. As articulated by the State Department’s Legal Adviser in 2016, the “international law of self-defense requires that such uses of force be necessary to address the threat giving rise to the right to use force in the first place.” As captured in the Caroline correspondence cited in that same speech which articulates fundamental principles of self-defense, the “necessity of self-defence” requires “no choice of means.” Therefore, in order for the use of force to be necessary, peaceful alternatives to addressing the supposed threat—including diplomacy—must be unavailable.
Contrary to the letter’s claim, the United States did not “make every effort to secure a diplomatic resolution” to address the potential threat posed by Iran’s nuclear program. Quite to the contrary, it was President Donald Trump who upended the diplomatic resolution to constrain Iran’s nuclear program when during his first term he withdrew the United States from the multilateral nuclear deal with Iran in 2018 that the Obama administration had orchestrated.
Further, the notion that “peaceful measures were exhausted” as claimed by the United States is also hard to swallow. To its credit, the second Trump administration appears to have pursued a new nuclear deal with Iran. But rather than diplomacy having been exhausted, public reporting suggests that it was abruptly truncated after the Trump administration discovered Israel’s own long-planned operation to attack Iran. Prior to the first Israeli strikes on June 13th, Trump indicated that he wanted to give nuclear talks with Iran another chance; indeed, a sixth round was slated for June 15th. (Moreover, contrary to the claim in the letter, the stumbling block in the nuclear negotiations with respect to enrichment was over whether Iran would be permitted to enrich uranium at all —something upon which the Islamic Republic insists and the U.S. rejected—rather than litigating over a specific threshold.)
When Israel’s military campaign began, the United States initially sought to distance itself, with Secretary of State Marco Rubio stating that “tonight, Israel took unilateral action against Iran. We are not involved in strikes against Iran and our top priority is protecting American forces in the region.” Trump still vocally held out hope for diplomatic meetings with Iranian officials within the week following the Israeli strikes. On Monday, June 16, he said, “I think Iran basically is at the negotiating table, they want to make a deal.” But as Israel’s tactical successes mounted, Trump’s public support for the campaign, and his touting of the U.S. role in making it happen, increased. The President’s decision to attack Iran appears to have been driven more by television coverage and advocacy by media figures, than any reasoned assessment that diplomatic alternatives had been exhausted.
In addition, the letter fails to specify what supposed armed attack by Iran on Israel provided the predicate for the use of force in collective self-defense. Instead, the United States asserts that for “almost forty-seven years the Islamic Republic of Iran has also waged a campaign of armed attacks against the State of Israel,” but does not specifically identify these putative armed attacks. The United States goes on to claim that these “attacks have intensified in recent years, including the launching of hundreds of ballistic missiles against Israel in 2024.”
The reference to Iranian ballistic missile attacks against Israel is a red herring for a few reasons. First, and most importantly, the U.S. airstrikes were not directed against Iranian ballistic missiles but instead were, per the letter, “directed solely against the Iranian nuclear program.” Second, the U.S. report omits that those Iranian ballistic missile attacks in April and October 2024 were themselves in response to prior Israeli attacks—against Iranian officials in Damascus and against a Hamas official in Tehran. (Clouding the picture is that the latter strike was also in response to Isreal’s killing the head of Hezbollah, Hassan Nasrallah in Lebanon.) The answer to the question of which State’s (if either) right of self-defense was triggered by those 2024 episodes is murky at best and hardly suffices to provide a predicate for the U.S. attack.
The letter also refers to attacks on Israel by Iran’s “proxies,” but it does not explain how, under principles of state responsibility or otherwise these attacks are attributable to Iran or relate to the attack on Iran’s nuclear facilities. One can try to invent theories for what the U.S. lawyers may have thought, but the letter itself is materially deficient. The conspicuous absence of an explanation in the Article 51 letter is telling.
Finally, nor does the Article 51 letter make out an argument that the United States was acting in anticipatory self-defense against an imminent threat of an armed attack against Israel. Although the letter devotes considerable space to describing the Islamic Republic’s enmity to Israel and misdeeds, it does not explain how any of this adds up to Iran’s nuclear program—the threat against which force was used—constituting an imminent threat of armed attack. Accordingly, that cannot be the legal basis either for the U.S. strikes on the nuclear facilities.
Legally Irrelevant Claims
To establish that its actions were consistent with the Article 51 of the UN Charter, the United States must show that its use of force was lawful self-defense or collective self-defense, that is the military action complied with jus ad bellum. Yet the United States also makes claims or alludes to other, irrelevant bodies of law.
Echoing Israel’s June 17th letter to the UN Security Council for its own attack on Iran, the U.S. letter situates Israel’s most recent strikes on Iran within an “international armed conflict.” Although it is certainly true that Israel and Iran (as well as subsequently the United States and Iran) were involved in an international armed conflict, the existence of such an armed conflict bears on the application of the law of armed conflict rules governing the conduct of hostilities once begun (jus in bello). The existence of such an “international armed conflict” does not bear on whether Israel or the United States resort to force was lawful as a matter of jus ad bellum including Articles 2(4) and 51 of the UN Charter.
In addition, the U.S. letter asserts that the “United States exercised the inherent right of collective self-defense (on behalf of Israel) and advanced vital U.S. interests in eliminating Iran’s nuclear program” (emphasis added). To the extent this second justification is intended as a legal one rather than a policy priority, it sounds more in the U.S. executive branch’s doctrine relating to presidential war powers (which gives the president some scope for the unilateral use of force in order to “advance a vital United States National interest”) than the international law of self-defense.
The Need for Proactive Executive Branch Lawyering
Taken together, the legal arguments put forward in the United States’ Article 51 letter are sufficiently unconvincing to leave the impression that they are post hoc justifications or otherwise immaterial or unrelated to whatever process led to the President deciding to attack Iran on the front end.
It would certainly not be the first time executive branch lawyers have been asked to develop a legal justification for the use of force after the fact. Indeed it is a more general feature of the present administration in which lawyers are regularly sidelined while policies are formulated or executive actions decided upon. Often it is only after the administration is forced to explain its actions in court that executive branch lawyers are consulted—and they sometimes struggle to justify them.
With respect to the use of military force by the United States, there is of course no realistic prospect for judicial review. But the absence of judicial review doesn’t alter the President’s own duties under the Constitution or the need for executive branch lawyers to help him fulfill those duties.
As I have previously argued, the UN Charter prohibition on the use or threat of force is not only binding on the United States under international law; it imposes domestic legal obligations on the President, as the President has a constitutional duty to “take Care that the Laws be faithfully executed” and the “Laws” encompass treaties, specifically including the UN Charter.
In order to aid the President in executing this constitutional duty, executive branch lawyers and other U.S. officials need to speak “law to power” to the President on this front. Given the current administration’s appetite for legal risk and the President’s seeming indifference to international law (perhaps most manifest in his articulation of expansionist aims vis-à-vis Greenland and the Panama Canal), those lawyers will struggle to be heard. Nevertheless, this is their function in any administration, and it is important not to lose sight of that amid the often chaotic news emerging from the executive branch. Ideally this would mean more than merely providing candid advice when asked or attempting to provide a legal justification after the fact. Presidential decision-making on such matters cannot simply be treated as the “weather”—something beyond human influence. Instead, it will likely require the President’s most senior advisors such as the Secretary of State, National Security Advisor, and White House Counsel to proactively intervene to explain to the President why the United States negotiated and agreed to be bound by the UN Charter—and the potential consequences of violating that treaty. (In addition, of course, to explaining the need for congressional authorization.)
Executive branch lawyers have rarely met this mark for proactive lawyering on use of force matters under any recent presidency, but that does not make the need for such lawyering any less relevant or urgent—particularly given the stakes involved.
